The present invention is directed to methods of manufacturing metal structures and, more particularly, to a method of heating and quenching a hollow metal member such as a bicycle crank arm during a manufacturing process.
Bicycle crank arms and other metal structures are often manufactured using a forging process wherein the crank arm is heated and quenched to strengthen the crank arm after a forging step. Problems arise when the heating and quenching steps are performed on hollow crank arms. More specifically, when a crank arm having a fully enclosed hollow space is heated, the air or other gas in the hollow space expands to a high pressure, thus causing deformation of the crank arm. One solution to this problem is to drill a hole on the side of the crank arm body to allow the gas to escape during the heating process. However, this requires an extra machining step, and the hole often allows the quenching liquid to enter the hollow space. The liquid in the space is difficult to remove easily and effectively, and it may cause future corrosion or oxidation. A cap or cover could be placed over the hole before the quenching step, but that would add additional steps, complexity and parts to the manufacturing process.